On Febr-16 2006, a heavy lightning strike occurred on a cooling water cell, whichis quit near the control room of Naphthacracker #3 (NAK3). This caused the so-called'black-screen' scenario for the ETBE unit. 'Black-screen' means thatoperators do not have control information of the process, although all controlvalves will remain in the last position. The cracker was shut downed because asmall gas leakage near the charge gas compressors. After some time, 2 majorproblems were identified:- Control equipment: all 30 dedicated compressor control devices weredefect or unreliable, most certainly caused by over-voltage induced by thelightning strike.Because of this defect, several bypass valves of the cracked-gascompressors were opened 100 %, leading to a mechanical problem:- Mechanical: 2 small cracks in 2 different (D25) weldolets of the flarevent lines of the charge gas compressors section 4e and 5e stage; causedby heavy vibration, induced by excessive high recycle flows and/orchattering of PSV's on the flare-header.The mechanical problem could be solved within 2 days. Because of lack of sparededicated compressor controllers, it was decided to build a simplified controlscheme in the DCS, for 1 set of each compressor system. Within 1 day, 1 set ofcharge-gas, C2 and C3 compressors were equipped with minimum flow andsuction pressure control. In a separate Hazop meeting, the reliability of the plantESD system, and the 'new' compressor DCS controls, was judged to beadequately safe. Only 60 hours after the lightning strike, the NAK3 plantproduced on-spec ethylene again.
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